Thursday, January 12, 2012

Managing Nuclear Waste thru Transmutation and More . . .

A first-of-a-kind reactor system has been set up in Belgium by coupling a subcritical assembly with a particle accelerator. The work is a major step in a program to research advanced waste management.

The equipment, known as Guinevere, is a demonstration model that supports the project for a larger version that will be called Myrrha (Multipurpose Hybrid Research Reactor for High-tech Applications). It was assembled by France's National Centre for Scientific Research and is managed by the Belgian Nuclear Research Centre (SCK-CEN) at Mol, about 50 kilometres east of Antwerp. The overall project is supported by 12 other European laboratories and the European Commission.

Nuclear terminology classifies an item of equipment as in a critical state if the chain fission reaction is self-sustaining and each reaction leads on average to one more. The term supercritical means the number of fissions is increasing, while subcritical means it is decreasing and will therefore dwindle to nothing. _World Nuclear News

Dangerous radioactive isotopes with long half-lives can be transmuted to elements with much shorter half-lives, using spallation neutrons. Spallation neutrons are generated when a beam of protons is accelerated into a spallation target. Neutrons, lacking a charge, do not have to overcome the "coulomb barrier", and can be much more readily incorporated into atomic nuclei to transmute one isotope into another.

It should be noted that the initial neutron source will be Deuterium - Tritium collisions. As the project builds steam, it will incorporate the proton beam - spallation target approach to generating neutrons.

Myrrha will be able to produce radioisotopes and doped silicon, but its research functions would be particularly well suited to investigating transmutation. This is when certain radioactive isotopes with long half lives are made to 'catch' a neutron and thereby change into a different isotope that will decay more quickly to a stable form with no radioactivity. If achievable on an industrial scale, transmutation could greatly simplify the permanent geologic disposal of radioactive waste. Myrrha can also be used to test the feasibility of lead fast reactor technology and is seen as complimentary to the Jules Horowitz Reactor, a thermal spectrum reactor under construction in Cadarache, France.

The total cost of Myrrha has been put at €960 million ($1.2 billion), with 40% of this coming from the Belgian government. SCK-CEN is looking to set up an international consortium to ensure additional financing and has completed a memorandum of understanding with the Chinese Academy of Sciences focusing on Myrrha. _World Nuclear News

Several kinds of nuclear fuel cycles are implemented today: most countries chose the so-called once-through cycle which basically considers spent nuclear fuel as waste, whereas others like France, UK, Japan and soon China reprocess their spent fuel to recover the energetically-valuable material Pu (and partially U) to produce Mixed Oxide Fuel (MOX) to be irradiated in a second cycle (a twice-through cycle). None of them allow a complete use of the natural resource; when discharged from reactor, 96% of spent nuclear fuel is still composed of U and Pu which can produce electricity and could be recycled.

Fast reactors

Although U-238 represents 99.2% of natural uranium, it is not fissile. It could be fertilised by neutron capture in order to produce Pu-239 which is fissile, and work with an implementation of Pu multi-recycling. This is however not possible in LWRs since neutron capture of U-238 is not efficient enough and the neutron capture of uneven isotopes of plutonium is high, leading to the formation of minor actinides. On the other hand, fast neutron spectra relatively increase the capture of neutrons by U-238, leading to the formation of plutonium isotopes which are all fissile in such conditions. For example, the ratio of the capture to fission cross sections of Pu-238, Pu-240 and Pu-242 are increased in fast spectra compared to thermal spectra by a factor of 22, 250 and 36 respectively. In conclusion, fast neutron spectra allow the effective consumption of U-238 to produce fissile plutonium isotopes which are subsequently fissioned to produce energy and electricity. Reactors using fast neutrons are hence potentially able to use more than 80% of the natural resources instead of < 1% for LWR. _ Much more including a look at transmutation nuclear waste management at WasteManagementWorld

Fuel recycling and nuclear waste management should be seen as integral to each other. Rather than wasting up to 99% of the energy in nuclear fuel as current LWRs can do, future generations of advanced reactors should be designed to utilise at least 80% of the energy -- thus extending the nuclear fuel supply of the planet by a factor of 80 or more.

Parenthetically, transmutation by the addition of a neutron is supposed to be behind the "cold fusion" or low energy nuclear reaction (LENR) efforts of a number of startup energy companies -- including Andrea Rossi's Leonardo Corporation, Defkalion of Greece, and Brillouin Energy. The methods being used by these startups for converting protons into neutrons is far from clear at this point.

In addition, sub-critical accelerator driven nuclear reactor designs have also been proposed for the use of thorium 232, an abundant fuel which is fertile rather than fissile -- it must be fed neutrons for conversion to fissile U 233, which spontaneously splits into smaller nuclei and more neutrons.

Parts of the above article were taken from an earlier article at Al Fin blog